The goal of this project is to characterize the defects in granulocyte development in the context of aging through analysis of progenitor cells. Aim 1 proposes to characterize the developmental stage(s) in which granulocyte development is aberrant in aged mice. Young and aged mice will be analyzed by means of 12 color combinatorial FACS analyses to deduce the discrete steps of granulocyte differentiation that are affected by aging. Cell surface markers specific for granulocyte development stages from the early stages of hematopoietic development to the granulocyte committed progenitors to fully mature basophils, eosinophils, and neutrophils will be assessed to distinguish where granulocyte development is affected. Also, granulocyte development will be analyzed during infection with the intestinal helminth, Strongyloides venezuelensis to analyze developmental potential under a physiologically relevant stimulus. Aim 2 will focus on verifying if the defects identified in aim 1 are cell intrinsic or extrinsic. Transfer of purified granulocyte progenitor populations from young and aged mice into aged and young congenic granulocyte-deficient hosts, respectively, will allow us to characterize the cell autonomous and/or microenvironment alterations that are manifested with age. To identify specific molecules that govern the cell intrinsic modifications identified above, Affymetrix microarrays will be used to compare the differential gene expression between progenitor populations from young and aged mice. This will allow us to identify specific transcription factors and signaling molecules that govern granulocyte fate decisions. Characterizing the extent to which these newly defined granulocyte progenitors are affected by aging, and the underlying mechanisms for such alterations, may help to identify points in this process that are amenable to therapeutic intervention.